Radioactive waste is generated by the operation of nuclear reactors, processing used nuclear fuel, the operation of particle accelerators, and other sources. A portion of this waste is in the form of a liquid stream that contains radioactive contaminants. The liquid waste must be processed to render it safe for disposal through solidification, radionuclide removal, and/or other methods.
Ion exchange is a common method used to treat liquid radioactive waste containing significant amounts of radionuclides. The ion exchange process involves moving the liquid waste stream through an ion exchange column filled with ion exchange media. The radionuclides in the liquid are absorbed by the ion exchange media and separated from the remaining liquid.
The amount of radionuclides absorbed by the ion exchange media increases as the process proceeds. The buildup of radiation emitting nuclides in the ion exchange media is significant and can produce dangerous radiation fields around the ion exchange column. In some situations, the ion exchange columns are enclosed in or surrounded by radiation shielding material to lower radiation levels and protect workers.
When the ion exchange media is loaded at or near its capacity, the ion exchange column is removed from service. The liquid waste stream no long flows through the column. In this state, the high radiation fields emitted from the radionuclides absorbed on the ion exchange media breakdown a fraction of the water contained in the ion exchange column through the process of radiolysis.
In radiolysis, the water molecules are split by the radiation and through a series of reactions form gaseous hydrogen and oxygen. The radiation can be in the form of alpha particles, beta particles, or gamma rays from radioactive isotope decay. The gases collect at the top of the ion exchange column and increase in concentration until they form a potentially flammable and/or explosive mixture.
The flammable/explosive gas mixture represents a significant hazard. It could ignite, damage the ion exchange column and ifs shielding, and spread the highly radioactive ion exchange media leading to potentially lethal radiation exposure to people in the vicinity.
Conventionally, this problem is addressed in one of two ways. The first is to connect the ion exchange column to a safety credited system that actively purges the hydrogen gas from the column. The second is to dry the ion exchange column to remove the water that is the source of the hydrogen. These systems must operate continuously to prevent the uncontrolled buildup of hydrogen. This makes them expensive to install and maintain because they require redundant systems with backup power supplies.